LED patterning (patterning sapphire substrate and LED epitaxial wafer patterning) has been considered as the most effective methods for enhancing light generation efficiency and light extraction efficiency and improving the quality of a light source (controlling the emitting direction of light and the uniformity of far field patterns) in academia and industry, namely so-called Nano-Patterned Sapphire Substrate (NPSS) and LED epitaxial wafer patterning technology (Photonic Crystal LED, Nanorod LED and Nanowire LED, etc.) is regarded as one of the most effective technical solutions for improving the light extraction efficiency and realizing ultrahigh-brightness LED in industry at present. Different from a flat clean silicon wafer used in the traditional IC field, a sapphire substrate and an LED epitaxial wafer have the characteristics of non-flat surface, warp, bow, large variation of thickness, sharp surface protrusions of several microns, relatively obvious surface defects and particle contaminants and fragility. Hence, it is extremely difficult to mass produce high-aspect-ratio micro/nano structures over large area at low cost high throughput and in large scale on the surface of the non-flat LED epitaxial wafer or sapphire substrate by adopting various existing micro/nanomanufacturing methods, and the requirement for LED patterning industrial-level application can not be met. For example, because the LED epitaxial wafer has the properties, e.g. warp, bow, surface waviness and sharp protrusions, the depth of focal for the traditional photolithography cannot adapt to the requirement of exposure over a large area; and when the large-area nanostructures are manufactured by using electron beam lithography, the cost is extremely high, the productivity is particularly low, and mass production is also extremely difficult. For NPSS, the existing contact or proximity lithography equipment cannot satisfy the resolution requirement. Although Stepper has been used to produce NPSS, the Stepper used in the semiconductor industry is too expensive for the LED industry, so that the manufacturing cost of LEDs is greatly improved. However, LEDs are especially sensitive to the production cost. In addition, some enterprises adopt second-hand renovated Stepper at present, which has problems on the aspects of product yield, equipment reliability and the like. Interference lithography has great advantages on forming periodic micro/nano structures over large areas, but the method has significant defects of poor selectivity of nanopatterns and severe requirement for a production environment (poor compatibility with an LED production process), and particularly, there is almost no commercial company which can provide a mature interference lithography machine (for patterning a full wafer) at present. Although other nanomanufacturing methods such as nanosphere lithography, anode aluminum oxide (AAO) templates, block copolymer self-assembly and the like have been attempted to be applied to the LED patterning, these methods show some limits, such as cost, productivity, consistency, yield, large-scale manufacturing and the like. The requirements of industrial-level production of the LED patterning with high throughput, low cost and good consistency cannot be satisfied.
Nanoimprint Lithography (NIL) is a novel micro/nanopatterning method which is based on the principle of mechanically modifying a thin polymer film (mechanical deformation of the resist) using a template (mold, stamp) containing the micro/nanopattern, in a thermo-mechanical or UV curing process. Compared with other micro/nanomanufacturing methods, NIL has the characteristics of high resolution, ultralow cost (the international authority assesses that the NIL of the equivalent manufacturing level is at least one order of magnitude lower than the traditional optical projection lithography) and high throughput, has the most significant advantage of manufacturing capability of large-area and complex three-dimensional micro/nanostructures, and particularly has the potential of implementing wafer-level nanoimprint on a non-flat (bow, warp or stepped) and fragile substrate by using soft UV-NIL process and the unique continuous nanopatterning capability for roll-to-roll NIL process. NIL has been considered as the most ideal technical solution for implementing large-area nanopatterning in academia and industry. However, the existing nanoimprint process applied to the LED patterning has many weaknesses on the aspects of mold life time, throughput, yield, reliability and the like, and particularly faces some challenging issues, such as difficulty in large-area demolding, deformation of a soft mold, damage to the mold due to particle contaminants and sharp protrusion defects, consistency and repeatability of imprinted patterns and the like. In addition, in the following process of a pattern transfer a material such as sapphire, GaN and the like is difficult to etch, and a hard mask layer generally needs to be deposited at first. To reduce the production cost and shorten the process route, a feature structure with high-aspect-ratio is directly imprinted on the resist, so that a hard mask layer procedure may be eliminated, the production process is simplified and the production cost is reduced. Hence, the LED patterning has a very urgent demand for some novel nanoimprint lithography technology for cost-effective mass production of large-area and high-aspect-ratio micro/nanostructures at low cost and high throughput on a non-flat substrates or curved surfaces or fragile substrates.
Therefore, to meet the industrial-level application requirement of the LED patterning and other large area nanopatterning, a new nanoimprint process and equipment need to be developed urgently, wherein the equipment has the capability of mass producing large-area and high-aspect-ratio micro/nanostructures at low cost and high throughput on non-flat substrate or surfaces.